Natural tannin-furanic thermosetting moulding plastics

Article abstract of DOI:10.1039/c3ra43095b

A new 100% biosourced thermosetting plastic material, named condensed tannin-furanic thermoset is prepared and characterized in this paper. This new material is synthesized from tannin and furfuryl alcohol, both of which are inexpensive plant-derived chemicals. The co-polymerization process of these two chemicals was studied by ¹³C nuclear magnetic resonance (NMR) and matrix-assisted laser desorption-ionization time-of-flight (MALDI-ToF) mass spectroscopy. The 100% renewable bioresourced tannin-furanic thermosetting resin was found to have a glass transition temperature as high as 211 °C, and a 95% weight loss temperature of 244 °C and 240 °C in a nitrogen and in an air atmosphere, respectively. The char yield is as high as 52%. Moreover, this new thermoset material shows excellent mechanical properties: a Brinell hardness of 23 HBS, which is higher than commercial acrylic, polyvinyl chloride, and a little lower than that of solid polystyrene. The compressive break strength was found to be as high as 194.4 MPa, thus higher than that of filled phenolic resins, and much higher than that of solid polystyrene and of acetal resins. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ra43095b

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Natural tannin–furanic thermosetting moulding plastics
Xinjun Li,a Arnaud Nicollin,a Antonio Pizzi,*a Xiaojian Zhou,a Alix Saugeta
and Luc Delmottebc
Cite this: RSC Advances, 2013, 3, 17732
A new 100% biosourced thermosetting plastic material, named condensed tannin–furanic thermoset is
prepared and characterized in this paper. This new material is synthesized from tannin and furfuryl
alcohol, both of which are inexpensive plant-derived chemicals. The co-polymerization process of these
two chemicals was studied by 13C nuclear magnetic resonance (NMR) and matrix-assisted laser desorption–
ionization time-of-flight (MALDI-ToF) mass spectroscopy. The 100% renewable bioresourced tannin–
furanic thermosetting resin was found to have a glass transition temperature as high as 211 uC, and a 95%
weight loss temperature of 244 uC and 240 uC in a nitrogen and in an air atmosphere, respectively. The
char yield is as high as 52%. Moreover, this new thermoset material shows excellent mechanical properties:
a Brinell hardness of 23 HBS, which is higher than commercial acrylic, polyvinyl chloride, and a little lower
than that of solid polystyrene. The compressive break strength was found to be as high as 194.4 MPa, thus
higher than that of filled phenolic resins, and much higher than that of solid polystyrene and of acetal
resins.
Received 19th June 2013,
Accepted 22nd July 2013
DOI: 10.1039/c3ra43095b
www.rsc.org/advances
chains. As is shown in Scheme 2, two parallel reactions occur
during step 1: a) the condensation of a hydroxyl group of one
Introduction
In recent years biosourced materials have attracted increasing
attention. Among these, tannin and furfuryl alcohol have been
widely used and researched. Condensed mimosa tannin
extract, obtained from the bark of a mimosa tree (Acacia
mearnsii, formerly mollissima, de Wildt), is predominantly
composed of prorobinetinidins,1–4 and profisetinidins
(Scheme 1). It can be hardened either by reaction with
formaldehyde or other aldehydes, or by induced autoconden-
sation. It has been used for over 40 years in industrial tannin–
formaldehyde thermosetting adhesives for wood products.5,6
Furfuryl alcohol (FA), the monomer of polyfurfuryl alcohol,
is the major chemical commodity produced from furfural.
Furfural is an aldehyde obtained from the hydrolysis of
pentosan-rich biomass such as agricultural residues of corn,
sugarcane, wheat, oat, cottonseed hulls, rice hulls, birch wood,
and hazelnut shells. Almost 85–90% of the furfural produced
worldwide is converted into furfuryl alcohol by a cheap
derivatization process.7 This furanic monomer can be easily
polymerized to polyfurfuryl alcohol through cationic conden-
sation.8–13 However, the characterization of the structure of
polyfurfuryl alcohol has always been a difficult task. The acid-
driven polymerization process is supposedly comprised of two
steps: 1) growth of linear chains and 2) cross linking of the
furfuryl alcohol molecule with the C5-hydrogen of another,
leading to sequence I; b) the condensation between two
hydroxymethyl groups, leading to sequence II:14–17
Moreover, as is shown in Scheme 3, sequence III is also
formed in step 1, which could explain the resin color change
observed during the self-condensation of pure furfuryl
alcohol.12,18
In step 2, structure III becomes the source of reticulation in
two ways:
a) Electrophilic substitution or condensation reactions of
cationically active growing chains at the sites associated with
conjugation (Scheme 4).
b) By Diels–Alder reaction between furan rings (dienes) in
oligomeric molecules and conjugated dihydrofuranic
sequences (Scheme 5).
Polyfurfuryl alcohol–lignin thermosets were prepared in
2010, however scanning electron microscope (SEM) images
showed that they always had a fractured surface and small
defects inside the resin.19 Tannin–furanic foams, prepared by
a tannin–furfuryl alcohol–formaldehyde reaction, have been
developed and widely researched.20–25 And tannin–furfuryl
alcohol has also been investigated as an adhesive in our
group.26 However, it is always difficult to obtain a solid, well-
cured, and non-foamed resin. In this paper, we have tried to
solve this problem with tannin and furfuryl alcohol to prepare
a 100% ‘‘green’’ solid resin to replace industrial petrol-derived
resins like phenolic resins in many applications.
a
´
LERMAB, University of Lorraine, 27 rue Philippe Seguin, BP1041, 88051 Epinal
cedex 9, France. E-mail: antonio.pizzi@enstib.uhp-nancy.fr
b
´
Institut de Science des Materiaux de Mulhouse, CNRS LRC 7228, 15 rue Jean
Starcky, BP 2488, 68057 Mulhouse, France
cContact information: ENSTIB-LERMAB, University of Lorraine, 27 rue Philippe
´
Seguin, BP1041, 88051 Epinal cedex 9, France
17732 | RSC Adv., 2013, 3, 17732–17740
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Scheme 3
Scheme 1
Experimental methods
Scheme 4
Materials
Furfuryl alcohol (with a purity of 98%) and para-toluene-4-
sulphonic acid (pTSA, with a purity of 97.5%) were purchased
from Acros Organics, Geel-Belgium, while mimosa tannin
(Acacia mearnsii, formerly mollissima, de Wildt) in the powder
form was supplied by Silva Team.
Preparation of the resin
Tannin and furfuryl alcohol were mixed well with a magnetic
stirrer. Then pTSA as a 65% water solution was added as a
catalyst under magnetic stirring for 2 min. Then the mixture
was left at 60 uC undergoing rotary evaporation under vacuum
for 0–2 h (pre-polymerization). After that the resins were left in
the oven at 60 uC for 2 days. Finally, the temperature of the
oven was raised to 100 uC for another 2 days. The formulae and
processes are shown in Table 1.
Scheme 5
Table 1 Formulae and the processes used for the preparation of the tannin–
furanic resins
FA (g) Tannin (g) pTSA (mL) Pre-polymerization 60 uC 100 uC
SEM
S1 10
S2 15
S3 15
10
5
5
0.40
0.38
0.38
2 h
0 h
1.5 h
2 days 2 days
2 days 2 days
2 days 2 days
The samples were cut into 5 6 5 6 4 mm3 pieces for
observation in a Hitachi S 4800 scanning electron microscope.
NMR and MALDI-ToF
Tannin–furanic resins prepared with different pre-polymeriza-
tion times were analyzed using 400.17 MHz 1H NMR as well as
100.6 MHz 13C NMR on a Bruker Avance 400 spectrometer. For
the 13C spectra, the signal was cumulated 10 000 times with a
pulse angle of 6.4 ms and 2 s recycling time.
The spectra of the reaction products of tannin with furfuryl
alcohol were recorded on a KRATOS Kompact MALDI AXIMA
ToF 2 instrument. The irradiation source was a pulsed
nitrogen laser with a wavelength of 337 nm. The length of a
single laser pulse was 3 ns. The measurements were carried
out using the following conditions: polarity-positive, flight
path-linear, mass-high (20 kV acceleration voltage), and 100–
150 pulses per spectrum. The delayed extraction technique was
used by applying delay times of 200–800 ns. The samples were
mixed with an acetone solution (10 mg mL21 in acetone) of the
matrix. 2,5-Dihydroxy benzoic acid was used as the matrix. To
enhance ion formation NaCl was added to the matrix (10 mg
mL21 in water). The solutions of the sample and the matrix
were mixed in the following proportions 3 parts matrix
solution, 3 parts polymer solution and 1 part NaCl solution,
and 0.5 to 1 mL of the resulting solution mix was placed on the
Scheme 2
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polymerization of furfuryl alcohol. The density of the tannin–
Table 2 Density and appearance of the solid tannin–furanic resins
furanic resin S3 is 1.36 g cm23
.
Density (g cm23
)
Appearance
To obtain more details the cured resins were studied by
SEM. The images are shown in Fig. 1a and b. In Fig. 1a, the
bubbles that can be seen inside of S2 are mainly in the range
of 70–120 mm. However, the SEM image of S3 did not show any
bubbles inside the material (see Fig. 1b). The digital photo in
Fig. 1c indicates that the new resin is easily molded.
S1
S2
S3
1.26 ¡ 0.02
1.31 ¡ 0.04
1.36 ¡ 0.01
Cracks
Bubbles
Good
MALDI target. After evaporation of the solvent, the MALDI
target was introduced into the spectrometer. The dry droplet
sample preparation method was used.
NMR
To investigate the reaction of tannin with furfuryl alcohol, a
sample of well-cured resin (S3) was scanned by 13C NMR. The
NMR spectrum obtained is shown in Fig. 2.
DSC, TMA and TGA
The peaks that are most indicative of a co-polymerization
reaction are the different kinds of –CH2– signals in the 30 to 40
ppm region. While this has the immediate appearance of a
single very broad peak, clear and distinct peaks are also
noticeable. Thus, the signals between 30–31 ppm belong to
methylene (–CH2–) bridges between two furan structures and
between a furan structure and the aromatic ring of tannin. The
signal at 38 ppm belongs to the unlinked C4 of the flavonoid
heterocyclic ring. The signal at 56 ppm is characteristic of the
hydroxymethyl group (–CH2OH) of furfuryl alcohol, indicating
that there is still some residual reactivity in the hardened
resin. The broad peak between 65 and 75 ppm belongs to the
C3 (peak at 72 ppm) of the flavonoid and to the carbohydrate
residues present in the tannin extract. The shoulder at 80 ppm
belongs to the flavonoid C2. There is a considerable super-
position of peaks in the 97–120 ppm range. Thus, the peaks at
104 ppm (a1) and 108 ppm (a2) belong to carbons of the
furfuryl alcohol ring. The peak at 110 ppm belongs to the C4–
C8 inter flavonoid linkage while the C4–C6 inter flavonoid
linkage signal is superimposed on the a1 signal of the furanic
ring. The shoulder at 115–120 ppm belongs equally to the
flavonoid C5 without an –OH and to the flavonoid C6 site
holding the –CH2– bridge coming from the reaction between
the tannin and the furfuryl alcohol. This wide peak hides
several other flavonoid peaks. The C1 peak of the flavonoid
appears clearly at 131 ppm. The peaks centered at 145 ppm
represent the C39 and C49 of the flavonoid while the shoulder
at 142 ppm is the signal of the unsubstituted site b1 of the
furan ring. The peak at 153–154 ppm is the superposition of
different signals, namely the C5 and C7 sites carrying phenolic
–OH groups (mainly C7 for the type of tannin used) of the
5–10 mg samples of the viscous prepolymer liquid (sample S3
before being cured in the oven) were sealed in hermetic
aluminum pans with lids. The polymerization process of the
pre-polymerized tannin–furanic resin was examined by a
differential scanning calorimeter (DSC 7, PerkinElmer) from
25 uC to 240 uC at a heating rate of 20 uC min21 in a nitrogen
atmosphere.
One piece of S3 (well cured) with dimensions of 17 6 4 6 6
mm was used in the thermomechanical analysis (TMA,
Mettler-Toledo DSTA840) from 25 uC to 300 uC with a heating
rate of 20 uC min21
.
A thermogravimetric analyzer (TGA) from NETZSCH (model
STA 449 F3) was used to study the thermal stability of the
cured tannin–furanic resin. The experiment was carried out
from room temperature to 900 uC using a heating rate of 5 uC
min21 under a nitrogen atmosphere.
Results and discussion
Density and appearance
The density and appearance of the products prepared
according to Table 1 are listed in Table 2. Densities of the
samples are measured according to ASTM_D792. The cracks in
S1 were caused by the unreacted tannin inside, and the
bubbles within S2 are due to the water produced during the
self-polymerization. The cracks and bubbles caused S1 and S2
to have lower densities than S3. So pre-polymerization is
necessary to vaporize the water that is produced in the linear
Fig. 1 Images of tannin–furanic resin: a) SEM image of S2; b) SEM image of S3; c) digital photo of molded products of the cured tannin–furanic resin.
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Fig. 2 CP-MAS 13C NMR result of tannin–furanic resin.
flavonoids and substituted the b2 site of the furan ring of
furfuryl alcohol.
the middle pH range (pH of about 11) they show a broad peak
and a separate sharp peak. This difference is supposed to be
caused by different reaction mechanisms. The polymerization
of furfuryl alcohol alone may also show two peaks depending
on the pH value.7,32,33 In Fig. 4, the DSC curve for a tannin–
furanic solid resin also shows a broad exothermic peak and a
distinct, sharp exothermic peak, the first from 53 uC to 145 uC,
and the second at 149 uC. It is likely that the first peak is due to
the linear self-polymerization of furfuryl alcohol and its co-
polymerization with the tannin. The second, sharp peak is
supposed to be caused by oligomers cross-linking.
Determining the glass transition temperature (Tg) of a
polymer is of interest. The glass transition temperature (Tg) is
defined as the maximum value of the loss tangent (tan d), and
tan d = E99/E9, where E99 is the loss modulus and E9 is the
storage modulus. The thermo mechanical analysis (TMA)
results in Fig. 5 indicate that the Tg of this resin is 211 uC.
However, for phenolic resins, even after modification by
benzoxazine and epoxy,34,35 or by silica hybrids,36 the Tg of
the resin is still lower than 180 uC. Moreover, the Tg of
polyfurfuryl alcohol is only 150 uC, and the Tg of polyfurfuryl
alcohol–lignin systems is 87–101 uC.19,37 This means that the
rigidity of the structure of the tannin molecule sharply
increases the Tg of the final material. This is a rather high
Tg value when compared to other polymers (polyurethane is
78–130 uC, polystyrene is 100 uC, polycarbonate is 145 uC, and
polyvinyl carbonate is 81 uC).38,39
MALDI-ToF
While the products of the reaction of tannin with formalde-
hyde are already well identified,27 the results of a MALDI-ToF
analysis of the sequence of products generated by the co-
polymerization reaction of furfuryl alcohol with mimosa
tannin are shown in Fig. 3 and Table 3.
Table 3 shows the different oligomers formed. All the peak
values are based on the MW of the species + 23 Da due to the
Na+ of the NaCl matrix used. Fig. 3a and b show peaks at 104
Da, 198/199 Da (Table 3), 215 Da, 365 Da and 397 Da,
indicating that the self-condensation of furfuryl alcohol does
indeed occur. The most common flavonoid repeating units
present in extracted mimosa tannins are fisetinidin, robine-
tinidin and delphinidin.1 The peak at 375 Da is an oligomer
formed by a fisetinidin monomer reacting with furfuryl
alcohol. And the oligomer at 658 Da is formed by a fisetinidin
dimer reacting with one furfuryl alcohol molecule. The whole
series of oligomers at 375 Da, 459 Da, 658 Da, 751 Da and 946
Da was identified as products of the co-polymerization
reaction between flavonoid monomers, dimers and trimers
characteristic of mimosa tannin with furfuryl alcohol and
furfuryl alcohol prereacted oligomers.
The reactions between tannin and furfuryl alcohol, are
summarized in Scheme 6.
TMA results for the tannin–furanic thermoset are shown in
Fig. 6. They indicate that under a nitrogen atmosphere the
sample reaches its 95% weight loss temperature at 244 uC (at
240 uC in air, data not shown here), and the carbon yield at 900
uC is as high as 52% (the value remained the same even when
Thermal analysis
For phenol–formaldehyde resins28–30 and phenol–urea–for-
maldehyde resins,31 when carried out at a high or low pH the
DSC curves of the curing process show two peaks. However, in
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